1. Field of the Invention
The present invention relates to a drive device for intake/exhaust valves (intake valves and/or exhaust valves) used in an internal combustion engine, more particularly relates to a valve drive device having a mechanism for changing the valve timing which enables free adjustment of the valve timing using hydraulic pressure.
2. Description of the Related Art
In a drive device of intake/exhaust valves of an internal combustion engine, there has been known a mechanism for changing the valve timing which is inserted into the connection portion between a timing pulley driven by a crankshaft and a camshaft and changes the phase of the camshaft with respect to the timing pulley by a cam mechanism controlled by hydraulic pressure or one, such as disclosed in the specification of German Patent No. 3909822, which does not use a camshaft for mechanically driving the intake/exhaust valves, but uses a high pressure oil pump having substantially the same construction as an in-line fuel injection pump for diesel engines to control and supply hydraulic pressure to hydraulic cylinders attached to the intake/exhaust valves and uses that hydraulic pressure to directly drive the operation of the intake/exhaust valves.
In the former related art, the only thing which could be changed by the valve timing changing mechanism was the phase of the camshaft with respect to the timing pulley, so there was the defect of a relatively low degree of freedom in control. In the latter, while the degree of freedom in control was relatively high, it was necessary to install a relatively large sized oil pump having the same number of cylinders as the number of cylinders of the engine, like with an in-line fuel injection pump, which made the construction of the related equipment complicated and was disadvantageous in the space and cost required.
As one means of solving the problems in the latter related art, consideration may be given to the use of a distributor type oil pump of a structure similar to a distributor type injection pump even in a hydraulic system for driving the intake/exhaust valves learning from the fact that a distributor type injection pump is used instead of an in-light fuel injection pump in relatively small-sized diesel engines.
In this case, however, the intake/exhaust valves of all of the cylinders of the engine are operated in succession by a single distributor type oil pump, which rotates at a speed half of the crankshaft of the engine, so in a four-cycle engine with four cylinders, for example, the time for opening the intake valve or exhaust valve of a single cylinder is even at the maximum 180.degree. in terms of the rotational angle of the engine crankshaft and cannot exceed 90.degree. in terms of the rotational angle of the oil pump. Of course, in an engine with a greater number of cylinders such as six cylinders or eight cylinders, the time for opening the intake/exhaust valve becomes much shorter. However, the time for opening the intake/exhaust valve of an internal combustion engine in general has to be about 220.degree. to 240.degree. in terms of the engine rotational angle (110.degree. to 120.degree. in terms of the rotational angle of the oil pump), so in this case it is not possible to secure the valve opening time required for an intake/exhaust valve and sufficient practicality for an internal combustion engine cannot be obtained.